The present invention relates to a method of controlling air/fuel ratio of an internal combustion engine, and to a system for practicing the method. More particularly, the present invention relates to such a method and device for controlling air/fuel ratio which provide either closed loop control or open loop control for said air/fuel ratio, according as to whether an oxygen sensor provided to the exhaust system of said engine is properly being kept warm by a heater which is built into it, or not.
It is known to fit an oxygen sensor to the exhaust system of an internal combustion engine. Such an oxygen sensor typically comprises a solid electrolyte or semiconductor, and varies a generated current or resistance in response to the concentration of oxygen in the exhaust gases of the engine. This electrical signal is fed to a control device which controls the amount of fuel provided to the engine in relation to the amount of air sucked thereinto, and is used for controlling the air/fuel ratio of the air-fuel mixture supplied to the engine by a closed loop or feedback process. Various such forms of control device, which practice various methods of air-fuel mixture ratio control, are per se known.
The output of the sensor element of such an oxygen sensor varies with temperature, and, particularly when the air/fuel ratio is weak and is in the range of 17 to 25, in order for the sensor element to accurately measure the oxygen concentration, said sensor element must be maintained at a temperature higher than a certain critical minimum active temperature. This maintenance of the temperature of the sensor element can be done by using a heater, and oxygen sensors with built in sensor element heaters have already been proposed, along with methods for operation of such heaters; for example in Japanese Patent Application No. 53-78476, which has been published as Japanese Patent Publication No. 54-13396. Further, in Japanese Patent Application No. 53-83120, which has been published as Japanese Patent Publication No. 54-21393, there has been proposed a method and a system for control of the electrical power supplied to such an oxygen sensor element heater, in which the power is varied as a function of intake manifold pressure, of throttle opening, and of engine revolution speed, so as to ensure that the oxygen sensor element is kept at a temperature no lower than its minimum active temperature.
The sensor element of such an oxygen sensor fitted to an exhaust system is of course heated up by the exhaust gases in the exhaust system, so the effect of a heater for the sensor element must be controlled to take account of the temperature of these exhaust gases. Now, in an internal combustion engine which is controlled by a throttle valve, the exhaust temperature is largely determined by the amount of air-fuel mixture supplied per engine piston stroke and by engine revolution speed, and if the air/fuel ratio of the air-fuel mixture is constant the amount of such mixture supplied is proportional to the rate of intake air flow. Therefore, in the above mentioned patent applications, the above are used as parameters, and the supply of electricity to the sensor element heater is varied depending on the engine load and the engine revolution speed. Thus, the exhaust temperature is considered to depend on the engine intake flow and engine revolution speed, and the values are determined experimentally in advance with reasonable accuracy. This method and system are adequate to keep the temperature of the sensor element of the oxygen sensor reasonably constant regardless of engine operational conditions, provided however that these engine operational condition do not change too abruptly.
However, when the battery voltage drops to too low a level, this method and system do not provide satisfactory operation. This is because, if a fixed or constant voltage is being applied to the sensor element, and also the temperature of the sensor element is kept constant, then the sensor current increases along with an increase in the oxygen concentration in the exhaust gases of the engine; while, on the other hand, if the voltage applied to the sensor element and the oxygen concentration in the exhaust gases remain fixed, while the temperature of the sensor element drops, then the current through the sensor element also drops. Accordingly, if the battery voltage drops so far that the temperature of the sensor element drops substantially due to the loss of heating power of the heater therefor, then, if closed loop feedback type air/fuel ratio control for the engine is still employed, the control system will think that the concentration of oxygen in the engine exhaust gases is lower than it really is, in other words that the air/fuel ratio of the engine is lower than it really is, and based upon this erroneous determination the control system will therefore control the air/fuel ratio of the air-fuel mixture supplied to the engine to be weaker than in fact it ought to be. This may bring the leanness of the air-fuel mixture for the engine beyond the region of combustibility, and may deteriorate the engine operation.
However, the temperature of the sensor element of the oxygen sensor does not directly correspond to the voltage of the battery source of electrical power for the heater element for the oxygen sensor. The reason for this is that the oxygen sensor as a whole has a certain thermal inertia, and therefore, if the battery voltage for a short transient period dips below an acceptable level, no problem will be caused with regard to the operation of the sensor element of the oxygen sensor, or to the control of the engine. Any system which aims at considering when the oxygen sensor element has cooled down so far as to not provide a reliable exhaust gas oxygen concentration signal must take into account this thermal inertia.
Yet further, the operation of any such system must be positive and stable; in other words, the system must not be prone to fall into any fluctuation condition. Also, since operation of the internal combustion engine in a closed loop feedback mode as described above provides much better air/fuel ratio control than any form of open loop control, such open loop control should only be performed to the minimum amount possible. This in any event is desirable in order to minimize fuel consumption, since such open loop control, in order to ensure satisfactory engine operability, typically provides a lower air/fuel ratio than does closed loop control.